1. Field of the Invention
The present invention relates to a microstrip type plane antenna to be utilized in microwave communication.
2. Description of the Background Art
A plane antenna has been developed as an alternative to a parabola antenna for microwave communication.
As an example of such a plane antenna, there is a microstrip type plane antenna shown in FIGS. 1(A) and 1(B), where the antenna comprises: a dielectric body 2; a grounding conductor body 1 provided on a lower side of the dielectric body 2; a current supply line 5 in a form of a strip line provided on an upper side of the dielectric body 2; and a patch shaped radiative element 4 provided on the upper side of the dielectric body 2 at an end of the current supply line 5. The antenna for practical use actually has a plurality of the patch shaped radiative elements 4 arranged in an array, among which the current supply lines 5 are arranged with their lengths, branching positions, and line widths appropriately adjusted for the sake of phase matching and impedance matching.
However, in this type of a microstrip type plane antenna, the current supply lines 5 are exposed on the radiation plane, so that there are unnecessary radiations radiated from branching and curving sections of the current supply lines 5. As a result, the radiation characteristic has been rather low in this type of a microstrip type plane antenna.
As an improvement on such a conventional microstrip type plane antenna, there has been a proposition of a microstrip type plane antenna shown in FIGS. 2(A) and 2(B), where the antenna comprises: a first dielectric body 2; a first grounding conductor body 1 provided on a lower side of the first dielectric body 2; a current supply line 5 in a form of a strip line provided on an upper side of the first dielectric body 2; a patch shaped radiative element 4 provided on the upper side of the first dielectric body 2 at an end of the current supply line 5; a second dielectric body 21 formed on the upper side of the first dielectric body 2 over the radiative element 4 and the current supply line 5; and a second grounding conductor body 11 provided on the upper side of the second dielectric body 21, which has a slot 3 at a position located directly above the radiative element 4.
In a microstrip type plane antenna formed from a plurality of such antenna units arranged in an array, the current supply lines 5 are arranged in a space sandwiched between the first and second grounding conductor bodies 1 and 11, so that the unnecessary radiations from the branching and curving sections of the current supply lines 5 tend to be suppressed by being obstructed by the second grounding conductor body 11 while the radiations from the radiative elements 4 can be transmitted without any obstruction through the slots 3 as shown in FIG. 2(C), such that the antenna efficiency can be improved.
Although this microstrip type plane antenna is effective in suppressing the unnecessary radiations from the current supply lines, it is also associated with a problem that the radiation efficiency of this antenna becomes low when the area of the slot 3 is small such that the gain is lower by 1 to 4 dB compared with the antenna of FIGS. 1(A) and 1(B). If the area of the slot 3 is increased, the gain may be improved, but the effect of suppressing the unnecessary radiations from the current supply line would be weakened such that the antenna gain becomes low.